The present invention relates to a zoom lens with large aperture and large zooming ratio.
Quite recently, zoom lenses with large aperture and large zooming ratio for 8 mm motion picture camera are being more and more used, whereby those lenses are generally designed in order to take pictures of bright objects principally out of doors.
On the other hand, however, along with the increase of the film sensitivity a camera which can take pictures under weak light in the room at night has also come to be used, which is the generally called XL cameras.
Hereby, it is desirable that the lens to be mounted on this kind of camera should have an efficiency proper for taking pictures of dark objects. The reason is that when a bright object is photographed the diaphragm in the lens is considerably closed, while when a dark object is photographed the diaphragm is nearly totally opened.
Hereby, it is well known that the efficiency of a lens more or less varies in accordance with the aperture value so that the lens is designed to have the best efficiency for the brightness with which pictures are taken most. Namely, the efficiency of the lens designed to have the best efficiency for bright objects somewhat goes down when pictures are taken with nearly totally opened diaphragm.
On the other hand, the focusing lens group of the zoom lens for a motion picture camera of small size is normally constituted of two positive lenses, one of which consists of a positive and a negative lens cemented with each other for preventing chromatic aberration. Hereby the cemented lens is so designed that there exists a difference between the refractive indices of the component lenses so as to prevent a divergent effect in order to check the spherical aberration of the first group in such a manner that there takes place little variation of the aberration during the zooming. Further it is already known that the two component lenses are separated from each other so as to form an air space between themselves in order to highly compensate the aberration by means of the space.